On account of its toxicity to fish, but above all on account of its contribution to the eutrophication of waters, ammonium is a substance which should be completely removed from wastewaters. Whereas there are numerous industrial processes for the removal of high concentrations of ammonium, the removal of ammonium in ppm concentrations can only be economically achieved by biological processes. This is done by two specialized groups of bacteria which derive their energy for the cell metabolism from the oxidation of ammonia to nitrite (ammonia oxidizers) and further to nitrate (nitrite oxidizers). The ammonia oxidizers use CO.sub.2 as the sole carbon source while nitrite oxidizers use additional carbon sources. The nitrate formed can be reduced to nitrogen by a number of heterotrophic bacteria at low oxygen concentrations and can therefore be completely removed (see for example (1)).
However, disadvantages attending the microbial elimination of nitrogen lie in the low growth rates of the nitrificants. CO.sub.2 is the sole carbon source so that cell growth is minimal. In addition, many organic compounds, such as isothiocyanates, amines, phenols and nitrogen-containing heterocycles inhibit the growth of ammonia-oxidizing bacteria. As a result, the biological elimination of nitrogen in industrial effluent treatment plants is seriously reduced.
Since nitrificants cannot be rapidly and quantitatively determined by microbiological culture methods, there have hitherto been no possibilities for recognizing changes in the quantity of nitrificants in wastewater populations and for achieving an optimal treatment capacity by corresponding control measures.